1. Field of the Invention
This invention relates to a lithium ion secondary battery comprising a positive electrode and a negative electrode facing each other via a separator retaining an electrolytic solution.
2.Description of the Related Art
There has been an eager demand for reduction in size and weight of portable electronic equipment. To meet the demand, it is essentially required to improve performance of secondary batteries used as a power source. In recent years various batteries have been developed, and improvements have been added thereto aiming for improved battery performance. Battery characteristics expected to be improved include voltage, energy density, resistance to high load, freedom of shape designing, and safety. Of currently available batteries, lithium ion batteries are the most promising secondary batteries for realizing a high voltage, a high energy density, and excellent resistance to high load and have been and will be given improvements.
A lithium ion secondary battery basically comprises a positive electrode, a negative electrode, and an ion conducting layer interposed between the electrodes. The rounded or angular lithium ion secondary batteries that have been put to practical use employ a positive electrode plate prepared by applying to an aluminum current collector a mixture comprising a powdered complex oxide of lithium and cobalt, nickel or manganese, a powdered electron conductor, and a binder resin; a negative electrode plate prepared by applying to a copper current collector a mixture of a powdered carbonaceous active material, such as graphite, non-graphitizing carbon or coke, and a binder resin; and an ion conducting layer made of a porous film of polyethylene, polypropylene, etc. filled with a nonaqueous solvent containing lithium ions.
FIG. 6 schematically illustrates a longitudinal section of a conventional cylindrical lithium ion secondary battery disclosed, e.g., in JP-A-8-83608. The battery shown in FIG. 6 has an electrode body 2 put in a case 1 made of stainless steel which also serves as a negative electrode terminal. The electrode body 2 is a roll of a laminate composed of a positive electrode 3, a separator (ion conducting layer) 4 impregnated with an electrolytic solution, and a negative electrode 5. In order for the electrode body 2 to maintain electrical connections among the positive electrode 3, the separator 4, and the negative electrode 5, it is necessary to apply external pressure thereto. For this purpose, the electrode body 2 is put into a firm case 1 to apply pressure for maintaining all the planar contacts. In the case of angular batteries, an external pressing force is imposed to a stack of strip electrode bodies by, for example, putting the stack in a rectangular metal case.
That is, a contact between a positive electrode and a negative electrode in commercially available lithium ion secondary batteries has been made by using a firm housing made of metal, etc. Without such a housing, the electrodes would be separated at their interface, and the battery characteristics would be deteriorated due to difficulty in maintaining electrical connections. However, occupying a large proportion in the total weight and volume of a battery, the housing causes reduction in energy density of the battery. Moreover, the rigidness of the case narrows the freedom of battery shape design.
Under such circumstances, development of lithium ion secondary batteries which do not require a firm housing has been proceeding, aiming at reductions in weight and thickness. The key to development of batteries requiring no housing is how to maintain an electrical connection between each of a positive electrode and a negative electrode and an ion conducting layer (i.e., separator) interposed therebetween without adding an outer force. A method comprising bringing electrodes and a separator into intimate contact by means of a resin and the like has been proposed as a joining means requiring no outer force.
For example, JP-A-5-159802 teaches a method in which an ion-conducting solid electrolyte layer, a positive electrode, and a negative electrode are heat-bonded into a unitary body by use of a thermoplastic resin binder. According to this technique, electrodes are brought into intimate contact with the solid electrolyte as an ion conducting layer over their whole area so that the electrical connections between electrodes and the solid electrolyte is maintained to perform the function as a battery without applying outer force. In general, however, ion conductivity, which is one of the physical characteristics governing battery performance, is much lower in a solid electrolyte than in a liquid electrolyte. It is therefore infeasible for the time being to obtain batteries which use a solid electrolyte and yet execute the same performance as conventional batteries using a liquid electrolyte.
In order to settle the problems associated with lithium ion secondary batteries using a solid electrolyte, JP-A-10-172606, etc. disclose batteries having a positive and a negative electrode adhered to a separator with a porous adhesive resin and having an electrolytic solution infiltrated into the positive electrode, the negative electrode, the separator, and the adhesive resin. In this type of batteries, since the positive electrode, the separator, and the negative electrode can be brought into intimate contact without applying external pressure, a lightweight bag can be used for packaging thereby to provide a lightweight battery having no rigid case.
A lithium ion secondary battery having the above-described electrode body in which the positive electrode, the separator, and the negative electrode are intimately adhered with a porous adhesive resin is produced by putting the electrode body in a packaging bag and vacuum-sealing the bag. There is thus provided a very thin and light lithium ion secondary battery. However, cases are sometimes met with in which the electrolytic solution vaporizes or decomposes to generate gas in use or during storage of the battery under some environmental conditions or some charging conditions. Because the contact between the packaging bag and the electrode body owes only to the pressure difference between the inside and the outside of the bag, an increase in inner pressure due to gas generation inflates the bag. It tends to follow that the bag is broken, and the electrolytic solution leaks out to damage the equipment.
In the light of the above problem, the present inventors have conducted extensive investigation to suppress deformation or breakage of a battery packaging bag due to gas which may generate on vaporization or decomposition of an electrolytic solution. It is an object of the present invention to provide a lithium ion secondary battery which is thin and light and yet the packaging bag of which is suppressed from expansive deformation due to gas generated in case of a temperature rise during storage or in use.
The invention provides a lithium ion secondary battery having an electrode body comprising a positive electrode made of a positive electrode active material layer joined to a current collector, a negative electrode made of a negative electrode active material layer joined to a current collector, a separator which retains an electrolytic solution containing lithium ions and is disposed between the positive electrode and the negative electrode, and a porous adhesive resin layer which retains the electrolytic solution and joins each of the positive electrode active material layer and the negative electrode active material layer to the separator, the electrode body being sealed into a packaging bag, wherein a resin film is provided between the electrode body and the packaging bag in intimate contact with the electrode body.